My understanding-

You must have to have a EAD card to start the job. The USCIS makes the decision for the OPT,

they will mail the EAD card to you. When you get the EAD card you can start working. Untill you get EAD card, it is illegal.

But, if the company still wants  you to start the work immidiately, may be there are some loopholes you can enjoy. It is just my guess though.